Conventional mechanized and automatic welding (and to a lesser extent brazing) practice has focused on methods for improvement in the joint microstructural condition and residual stress level, especially for materials susceptible to stress-induced cracking such as stress corrosion cracking (SCC). In addition, emphasis has been placed on improving the joining productivity while maintaining or increasing the joint quality, especially for thicker section materials. One of these modifications, relative to conventional V-groove joints, has been to decrease the volume of the filler deposited by reducing the width of the weld joint. This technique is known in the art: as "narrow groove" (or narrow gap) welding. As the joints are made thinner with steeper side wall angles, there are width and aspect ratio limitations on the joint design which can be reliably completed, even when using only a single filler material. As the technical and practical needs increase to make joints even thinner, the difficulty of locating and precisely controlling the feeding of multiple, nonparallel filler materials into these narrow and relatively deep joints using conventional equipment and procedures becomes even greater, or is impractical for many applications.
An additional problem for thin, high-aspect-ratio joints is the limitation in the filler deposition rate and corresponding joint completion rate, which are strongly controlled by the maximum practical filler melt-off rate that does not result in risk of lack of fusion or other defects. The practice of feeding only a single filler into the molten pool at any point in time during the deposition of a filler pass is inherently limited in its thermal efficiency for utilizing the most power of the heat source. The feeding of two fillers simultaneously, one of which is fed into the molten pool but intentionally not located in the hottest or most effective melting portion of the heat source, is also inherently limited in thermal efficiency. These practices result in undesirable limitations on the filler melt-off rate and productivity.
Commercial systems are available for feeding multiple filler wires. The general approach used in the welding industry for multiple filler material addition is to feed using two nozzles, each feeding at different times. The nozzles are aimed from different directions, typically from the leading and trailing sides of the torch (or other heat source), with respect to the direction of torch travel. One scheme is to feed from the two opposing, non-parallel nozzles alternately as the direction of torch travel is periodically changed from a forward to a reverse direction, such as to continue an orbital joining application while rewinding cables which have become wrapped around a component while traveling in the forward direction during the deposition of multiple fill passes. This commercially available system configuration is commonly called "dual wire feed" and allows a productivity improvement for some multi-pass, bidirectional travel applications.
Another known scheme is to feed from two opposing, non-parallel nozzles simultaneously while welding in either the forward, reverse or both directions, typically in an attempt to improve the filler deposition rate. One variation of this scheme is to try to align both filler nozzles, and therefore the aim points of both wires, to the desired part of the molten pool (under the heat source).
Another variation used with lateral torch and filler material oscillation is to synchronize the aim of one filler nozzle to the current position of the heat source, and to synchronize the other nozzle to be aimed into the portion of the molten pool from which the heat source has just moved in an effort to utilize some of the excess/residual heat remaining in the pool. In this latter configuration, the "chill" filler material feed rate typically is only a small fraction of the primary feed rate. This system is claimed to improve productivity by the use of the additional out-of phase trailing-side chill wire feed.
A number of welding systems are commercially available which allow pulsing of a single filler material between two feed rates synchronized with pulsing of the arc between two power levels. At higher pulse frequencies, however, the combination of mechanical slack in the drive mechanism (motor gearheads, etc.) and the clearance between the inside dimension of the filler conduit and the outside dimension of the filler material cause the individual feed rates to be smeared into an average value as the filler leaves the outlet end of the feed nozzle. Effectively, this averaging condition is aggravated by the mechanical inertia of the drive mechanisms, and results in inefficient use of the significantly greater filler material heating and melting capability of the higher power level. The heating and melting capability of an electric welding arc, for example, is proportional to the square of the current, so that high current levels are significantly more effective in melting filler material than lower current levels.
Conventional filler nozzles are stiff and, due to their large width, cannot be inserted into a very thin joint. The standard approach of increasing the filler stickout beyond the end of the nozzle in order to reach into a thin joint is limited by the lack of filler position control near the bottom of such joints if they are deep, as is the case in thicker materials. This lack of position control not only leads to filler melting inefficiencies as the aim to the hottest part of the arc is degraded, but also leads to electrode contamination, fusion defects, and process terminations when the filler material inadvertently contacts the (nonconsumable) electrode and disturbs the arc geometry and thermal properties.
Multiple filler material equipment designs utilizing individual nozzles for multiple feed applications use straight guide tubes which do not automatically compensate for the fact that the unsupported filler shape is not straight, and that the end does not follow a straight path after leaving the nozzle. This design has the disadvantage of providing no aiming control of the wire position after it leaves the outlet end of the nozzle, to compensate for the fact that the wire has a "cast" or helical configuration remaining from the permanent bending that occurs as it is wound on circular spools. The previously bent wire springs back into the curved configuration, reflecting a portion of the bending strain it had when on the spool. This curvature is typically accounted for as the filler is initially positioned relative to the heat source (such as the tip of a non-consumable electrode), and in some cases can be manually overridden during the course of the joining with the use of multi-axis motorized filler nozzle positioners. This method relies on an operator for periodic aiming adjustments, and would be very tedious when more than one filler is fed at the same time, especially with high-speed joining practices.